Flash and laser lamps are generally constructed from a tube of fused silica/quartz opposite ends of which contain metal electrodes to which electrical operating power is supplied via conductive supports, which also serve to mount the lamp in a lamp holder, when in use.
Due to the different co-efficients of expansion of metal and fused silica/quartz, special materials have been developed, to interpose between the metal conductive supports for the electrodes and the tube wall of such lamps, to accommodate the differential rates of expansion, as the lamp increases and decreases in temperature in use. Typically the electrodes are constructed from Tungsten and an intermediate sleeve of a glass like material having an appropriate coefficient of expansion, such as a seal glass such as GS10, is formed around the Tungsten rod before it is introduced into and sealed to an end of the lamp tube. Sealing glass is supplied inter alia by GB Glass Ltd., and GS10 sealing glass as supplied by GB Glass has been used with quartz and tungsten combinations.
As used herein the expression GS is intended to mean any suitable material which can be bonded to a metal electrode and likewise fused to fused silica/quartz materials and whose coefficient of expansion is such as to accommodate the generally greater expansion of metal (for a given temperature rise), than is produced in fused silica/quartz by the same rise in temperature. GS10 seal glass is an example of a GS material.
The constructional steps of the known method leading to the formation of a glass to metal seal at an end of a fused silica/quartz tube are as follows:
(1) A Tungsten electrode is prepared to receive a sleeve of GS seal glass, by heating and rotation about its length axis PA1 (2) A stick of GS is also heated and as the end becomes molten, it is brought into contact with the rotating heated Tungsten rod support which extends axially from the Tungsten electrode so that molten glass becomes attached to and "smeared" over the surface of the rod to form a relatively uniform thickness sleeve over approximately 1-2 cms of the length of the rod; PA1 (3) The central region of the sleeve is increased in thickness by reheating it and a GS stick, and while the sleeved rod is rotated, touching the end of the glass stick against the central region of the sleeve to cause an annular build-up of GS to occur. The step is commonly referred to as "spinning a bead" onto the sleeve; PA1 (4) Next a length of fused silica/quartz tube, cut to the desired length of the lamp housing, is heated at one end whilst being rotated around its length axis, and the heated end is closed by spinning a bead of molten GS into and over the heated end of the tube. (The GS10 stick is of course heated before it is brought into contact with the heated end of the tube); PA1 (5) One end of a smaller diameter tube of fused silica/quartz is then heated, the interior of the lamp tube is pressurized with a non-oxidising gas, typically and usually nitrogen, and a region of the wall thereof is heated until soft to permit the heated end of the smaller diameter tube to be pushed therethrough and fused thereto, so as to extend radially as a side tube therefrom. By pushing the end of the smaller diameter side tube through the locally heated, softened region of the lamp tube wall, the interior of the latter communicates with the interior of the side tube, and this communication is maintained by maintaining a positive gas pressure in the lamp tube whilst the fusing is completed. After this the heating is removed; PA1 (6) The end of the radially protruding side tube which has just beep added is now closed by heating the outboard end thereof to collapse the side tube wall; PA1 (7) The previously closed end of the fused silica/quartz lamp tube is now reheated, and the internal pressure of the assembly of tubes is increased, so as to cause the GS10 dome which has closed the heated lamp tube end, to balloon axially and puncture. PA1 (8) Whilst rotating the lamp tube and keeping the punctured end hot and near molten, a carbon tool is introduced into the punctured end and the diameter of the opening in the GS dome is made concentric with the lamp tube axis and enlarged so as to be capable of receiving the electrode; PA1 (9) The electrode and its integral sleeved rod is now introduced axially into the opened end of the lamp tube whilst the latter is rotated, until the annular bead makes contact with the end of the lamp tube. Both are reheated until the GS becomes molten and can be worked, using a carbon tool, so as to cause the ring of GS defining the open end of the lamp tube to become merged with the GS10 bead on the Tungsten rod, and the GS material to become fused into a uniform annular seal. PA1 (1) reheat the GS dome closing off the end of the tube and remove the excess GS material so as to leave an annulus of GS around the end wall of the opening in the quartz tube; PA1 (2) insert the electrode and protruding Tungsten rod until the GS bead previously spun around the rod is near to the GS annulus at the end of the quartz tube; PA1 (3) heat the quartz tube and work the end thereof down to form a frusto-conical end so that the annulus of GS is now only just greater in diameter than the GS sleeve around the Tungsten rod protruding therethrough; PA1 (4) axially move the electrode and rod so as to bring the GS bead on the rod into contact with the annulus of GS around the now frusto-conical end whilst maintaining a positive gas pressure within the lamp tube at least until the bead touches the annulus; PA1 (5) momentarily balancing the pressure between the inside and outside of the lamp tube after the bead has fused to the annulus whilst using a carbon tool to work the quartz tube down onto the bead and cause it and the annulus of GS material to become more completely fused; PA1 (6) increasing the internal gas pressure within the lamp tube assembly to obtain a smooth internal concave surface to the fused GS material, and PA1 (7) allow the assembly to cool and then locally reheat the assembly using a movable, local source of heat, and move the heat source axially along the tube, to stress relieve the assembly. PA1 (1) fitting the partially completed lamp tube into a rotatable chuck with the closed end inside the chuck; PA1 (2) fitting to the side tube protruding from the lamp tube a gas line communicating between it and a rotatable manifold, to convey gas to the inside of the lamp tube via the side tube; PA1 (3) closing off the open end of the lamp tube in the manner previously described by means of a dome of GS material; PA1 (4) reheating the GS dome and removing excess material to leave an annular ring of GS material around the end of the lamp tube; PA1 (5) introducing a Tungsten electrode having an integral axial support rod into the open end, having a previously formed GS bead around the rod, in manner known per se; PA1 (6) heating the end of the lamp tube and, using a carbon tool, working the tube end down to form a frusto-conical end so that with axial movement of the rod relative to the tube to facilitate the merging of the bead and the annulus of GS, the latter can become fused to the bead of CS on the rod; PA1 (7) momentarily balancing the pressure as between the inside and outside of the now closed tube and, using a carbon tool, working the external surface of the lamp tube down onto the bead to cause the GS material to more completely fuse and form a good glass to metal seal between the rod and the lamp tube; PA1 (8) increasing the internal gas pressure within the tube to ensure a smooth internal concave surface to the GS seal at the end of the lamp tube; and PA1 (9) cooling the assembly and thereafter applying a source of heat locally at spaced apart positions along the length of the assembly to stress relieve the tube and the region of the GS seal.
A lamp requires a similar arrangement at the opposite end, and the appropriate steps may be repeated at the opposite end of the lamp tube to enable a second electrode to be sealed in a similar manner into the said opposite end.
Final assembly of a lamp involves evacuation of the lamp tube assembly and usually the introduction of a specific gas usually at low pressure, via the side tube, which is then finally closed off and sealed by heating.
Lamps constructed in accordance with the above method have been found to possess a weakness in the end regions thereof where a GS to GS seal has been formed. Investigations have indicated possible reasons for this weakness and it is an object of the present invention to provide an improved method which reduces the chance of weakness being introduced into the structure by the manufacturing process.